1. Field of the Invention
The present invention relates to submarine optical fiber cables and in particular, to cable constructions for protecting the interior construction of the cable from sea water ingress into the cable.
2. Description of the Prior Art
It has been attempted to use optical fibers as transmission media of submarine cables.
A submarine optical fiber cable which has already been proposed includes an optical fiber unit comprising at least one optical fiber core and a tension member which are covered together with a buffer layer of an insulating material in accordance with necessity, as disclosed in UK Pat. No. GB 2017968 B. The optical fiber unit is enclosed in a pressure resisting sheath for protecting the optical fiber unit from the external pressure. The pressure resisting sheath is covered with an outer water-proof jacket comprising a layer or layers of insulating materials.
In such a cable, if the pressure resisting sheath and the outer jacket are damaged unfortunately during or after being laid, sea water enters, or leaks into, the pressure resisting sheath so that the cable cannot be repaired but must be replaced by a new one. In order to prevent the sea water from flowing over the entire length of the cable, it is known to provide dams in the pressure resisting sheath at suitable intervals, for stopping the introduced sea water from flowing longitudinally therein, as shown in FIG. 4 of the above-described UK Patent.
In a certain construction of optical fiber submarine cables for deep sea, the pressure resisting sheath is composed of an inner and an outer metal tube coaxial with each other and tension wires extended in an annular space between the inner and the outer metal tubes. In such an arrangement, even if such dams are provided in the inner metal tube, the introduced water flows along the tension wires over the entire length of the cable.
Further, if there is any opening, pin hole or other production fault in the inner metal tube, the introduced sea water also enters the inner tube and longitudinally flows to wet the optical fiber core.
When a submarine optical fiber cable is damaged or broken after being laid, it is necessary to hang up a cable section having the faulty point onto a ship for repairing the broken point. However, it is practically difficult to measure a distance from the hanged-up portion of the cable to the faulty point along the cable.